delete_packet_filters -> Delete Bearer via async Gx CCR-U (#22 Increment 2)

[next-nf]

Stacked on #49 (base async-ue-bearer-22). Second increment of #22 (UE-requested bearer resource modification).

Turns a UE Bearer Resource Command carrying delete_packet_filters into the first real diameter-async async_m procedure: report the removed SDF filters to the PCRF over a Gx CCR-Update, await the reply non-blockingly, apply the PCC delta, and when the bearer's last rule is gone run the single-bearer deactivation echoing the PTI. On reject → Bearer Resource Failure Indication.

Commits

• encode Packet-Filter-Operation/Packet-Filter-Information in the Gx CCR (DELETE shape: operation + identifiers only, per TS 29.212 §4.5.2)
• smf_tft:pf_ids_to_sdf/2 reverse lookup — inverts #ded_bearer.sdf_to_pf, fails loud on a non-injective map or unknown id (never guesses)
• the async procedure + delete_packet_filters reject-arm rewire + smf_aaa_pcf:merge_ctx/2
• gate refinement: postpone fire-and-forget Gx {'$reply'} while an async procedure is parked — closes a pcf/aaa_session write-back race (an early Step-5 gate refinement)
• TODO(PGW: implement UE-requested bearer resource modification (TAD add/replace/delete) #22) note on the remove-only fold (Increment 3 adds the install pass)

Design notes

• Empty-TFT → Delete Bearer is judged post-fold via smf_gsn_lib:detect_removed_bearers/3 (the PCRF's rule decision governs), not from the TAD op.
• fold_cca's stripped #pcf_ctx{} (app_id=undefined) is merged via merge_ctx/2, never wholesale-replaced (would break the next CCR-U).
• Correlation (PTI/EBI/tunnel) rides the do-block closure across the await — no separate map.

Tests

pgw_SUITE 129/129 (2 new e2e cases: delete → one Delete Bearer echoing the PTI; reject → Failure Indication). diameter_Gx_SUITE 7/7, smf_tft_SUITE 25/25. Default compile green.

Out of scope (Increment 3)

The Update-TFT outcome + PTI Update emitter + uplink-filter guarantee; add/replace; requested-QoS.

Follow-ups

• handle_cca reject/failure routes badrecord on the async static mock's undefined #state{} #50 — the async static mock can't drive a PCRF-Result-Code reject e2e (handle_cca badrecords on its undefined state); the reject case substitutes an unknown-filter trigger exercising the same br_delete_err path.
• async error callbacks routing through handle_ctx_error/4 don't drain async_pending -> coarse gate can wedge #51 — pre-existing dormant landmine: async error callbacks routing through handle_ctx_error/4 don't drain async_pending; unreachable today, doesn't affect this PR (br_delete_err drains correctly).

[next-nf]

Superseded by the async-invoke restack. This increment was re-authored onto invoke(pipeline_async) and collapsed into #56 (delete/add/replace together) on the redone #48 foundation. Closing in favor of #56.